The observer-expectancy effect, in science, is a cognitive bias that occurs when a researcher expects a given result and therefore unconsciously manipulates an experiment or misinterprets data in order to find it. Because it can skew the results of experiments (especially on human subjects), double-blind methodology is used to eliminate the effect.

Observer-expectancy effect is often a cause of "odd" results in many experiments, notably in paranormal investigations. One famous example was the horse Clever Hans, who seemed to be performing arithmetic and other amazing skills, but in reality took cues transmitted unconsciously by his trainer and observers. In another experiment, children were given laboratory mice and told that some were bred for intelligence, some for dullness. In reality, the rats were chosen at random, but the children reported that the "smart" rats learned mazes faster than the "dumb" rats.

Another example of the observer-expectancy effect is demonstrated in music backtracking; some people expect to hear hidden messages when reversing songs, and therefore hear the messages, but to others it sounds like nothing more than random sounds. Other prominent examples include facilitated communication and dowsing.

The observer-expectancy effect is also called the experimenter-expectancy effect, observer effect, or experimenter effect.